Recent advancement in electrophotographic systems has brought a rapid increase in the demand for electrophotographic devices such as copy machines and laser printers and has also created the need for higher performance of these devices.
According to conventionally known methods and devices for full color electrophotographic images, an image is obtained by forming a latent image based on color image information on a latent image carrier such as an electrophotographic photoreceptor; forming a toner image using color toners corresponding to the colors of the latent image; and transferring the toner image to a transfer material. This image formation process is performed repeatedly. Then, the toner image on the transfer material is thermally fixed to produce a multicolor image.
For these processes to run smoothly, it is firstly required that the toner maintains a stable electrostatic charge level, and it is secondly required that the toner has good fixability to paper. In addition, the devices include heating elements in their fixing sections, and these heating elements raise the temperature in the devices. Thus, it is also required that the toner does not undergo blocking in the devices.
Further, there is a demand for further miniaturization, higher operation speed, and better image quality performance of electrophotographic devices, and to reduce the amount of energy consumption in a fixing step. Thus, there is a strong demand for improving low-temperature fixability of the toner in order to save energy.
In addition, recently used transfer materials include various types of paper including recycled paper with a rough surface and coated paper with a smooth surface. In order to handle surface properties of these transfer materials, fixing devices with a large nip width, such as soft rollers and belt rollers, are preferably used. However, a larger nip width results in an increased contact area between the toner and fixing rollers. This causes so-called “high-temperature offset phenomenon” in which the fused toner is attached to the fixing rollers. Thus, offset resistance is a prerequisite.
In addition to the above, much higher gloss is required for multicolor images (full color images) than black-and-white images (monochrome images) due to processes such as reproduction of images such as photos. It is necessary to ensure that the resultant multicolor images have a smooth toner layer.
Thus, in forming a toner image, a toner is required to exert low-temperature fixability and offset resistance. Also, a formed toner image is required to exhibit high gloss. In addition, the demand is increasing for a highly glossy toner image that can be obtained in a wider working range.
Toner binders have a great influence on toner properties as mentioned above. While known resins for toner binders include polystyrene resin, styrene-acrylic resin, polyester resin, epoxy resin, polyurethane resin, and polyamide resin. Polyester resin has recently attracted particular attention because the balance between storage stability and fixability can be easily achieved with the polyester resin.
Patent Literature 1 suggests a toner containing a reaction product of a mixture of a resin for producing macromolecules, a polyester resin and an isocyanate.
However, even if this method can prevent the high-temperature offset phenomenon to some extent, it is difficult to achieve low-temperature fixation because the lower limit fixation temperature is also raised simultaneously. In addition, since a urea group and a urethane group derived from isocyanate are highly cohesive, the resin has very poor grindability. The resin also has poor homogeneity and poor heat-resistant storage stability. Thus, with regard to color toners, the demand for higher operation speed and lower energy consumption has not been fully met.
Patent Literature 2 suggests a toner containing a product obtained by crosslinking an unsaturated double bond-containing polyester resin using a radical reaction initiator.
However, while the grindability of the resin is improved by the technique disclosed therein, the glossiness is insufficient. Thus, the demand for higher image quality, higher operation speed, and lower energy consumption has not been fully met.
A smooth toner layer is required to provide a highly glossy image. One of known physical properties that control the smoothness of the toner layer is the loss tangent tan δ (G″/G′) represented as the ratio of storage modulus (G′) to loss modulus (G″) of dynamic viscoelasticity of a binder used in the toner.
In order to provide a color toner for improving glossiness, Patent Literature 3 suggests a method for controlling the loss tangent tan δ using a hybrid resin including a polyester unit and a vinyl-based copolymer unit. While this method provides excellent glossiness and fixability, the image strength of images obtained by fixing the toner is insufficient.
As described above, conventional techniques have not been able to provide toner binders or toners applicable to full color images, which exhibits offset resistance and can provide high gloss for a toner image, and which are excellent in low-temperature fixability, grindability, image strength, and heat-resistant storage stability.